For the pressurized water reactors, yes, we call the water going into the core "cold" but it is actually very hot (about 550F or 287C); the water is heated up as it passes through the core and exits 'very' hot (about 610F or 320C). These temperatures are when the plant is running at full power. When the reactor is shutdown (like for refueling) the temperatures are much lower (below boiling point at atmospheric pressure).

In a BWR, the steam is passed to the turbines, actually the high pressure turbine, then through a moisture separator, then to the low pressure turbines. The discharge from the LP turbine goes to the condenser. The condensate from the HP and intermediate stages is sent to reheaters that increase the water from the condenser, and that flow becomes the feedwater. The feedwater is feed back to the reactor pressure vessel and flows into the annular region between the core shroud (which surrounds the core) and the reactor pressure vessel. In that annulus, the colder feedwater is mixed with the condensate from the moisture separator and steam dryer, which is at the saturation temperature of the core outlet. That combined flow is pumped to the core, either by jet pumps (GE) or direct impeller pumps (ABB, Siemens).

The reactor inlet water is slightly below saturation (i.e., somewhat subcooled). The coolant quickly reaches saturated liquid condition in the core - in the first meter of core length. The rest is a mix of liquid and steam and boiling regime transitions from nucleate to bulk boiling.

The NRC diagram is very simplistic.

Temperature differences in a liquid in a heat exhanger depend strongly on the mass flowrate of the hot and cold fluids. It is the thermal energy that is transferred, and one equates the products of the flow rates and respective changes in specific enthalpies.

The Reactor Core is made of a device in the middle that holds the Uranium Rods and inbetween eatch Uranium Rod is a Boron Rod.

Not quite. In most LWRs (light water reactors), the fuel rods (UO2 or (U,Pu)O2 ceramic pellet surrounded by Zr-alloy cladding tube) are maintained in regular square lattice, or in VVER (Russian) fuel, a triangular lattice. The control rods may contain neutron aborbers suchs boron, AIC (silver-indium-cadmium), hafnium or dysprosium. In PWRs (pressurized water reactors), the control rods are fully withdrawn from the core, except for the tips which sit in the top of the guide tubes. In a 17x17 lattice, there are 264 fuel rods and 24 control rod fingers. The control rod fingers (rodlets) are suspended from a structure called a spider. The spider is attached to a hub, which is attached to a control rod drive shaft, which is attached to the control rod drive mechanism.

In BWRs (boiling water reactor), the control rods (blades) have a cruciform shape and each sit among a set (cell) of four assemblies. BWRs use a subset of control rods fully or partially inserted during the cycle.

Then this whole Core is filled with water so all the Rods are submarged.

The water is used for cooling and moderation. In PWRs, the water is liquid. In BWRs, water is boiled.

Then to turn the Reactor on they Lift the Boron Rods out from inbetween of the Uranium Rods.

More or less. In PWRs, the control rods are withdrawn from the top, and drop in under gravity if necessary to shut the reactor down quickly. In BWRs, the control rods are hydraulically inserted from below. To achieve criticality, where at least one neutron from each fission survives to cause a (subsequent) fission, the amount of non-fuel neutron absorber is continually adjusted.

The Boron Rods absorb the Uranium Radiation so when they Lift the Born Rods out the Uranium Radiation can pass to the other Uranium ROds and make the Uranium Reaction grow.

The boron (or hafnium, AIC, dysprosium) absorbs neutrons. Absorbing more than a certain number of neutrons such that there is less than one neutron for each fission to cause fission in the fissile U or Pu will cause the reactor power to decrease. To shutdown a reactor, the control rods are usually fully inserted.

We do not use the term, uranium radiation, but neutrons, beta particles or gamma rays. Neutrons cause fission, but beta particles and gamma rays, which represent part of the fission energy, do not cause fission.

The majority (~85%, or ~170 of 200 MeV per fission) of fission energy is released as kinetic energy of the nuclei of the fission products. The released neutrons, gamma rays, and beta particles represent the rest. Some energy is released as anti-neutrinos (from beta decay of fission products and some transuranics) which is not recoverable.

I am looking at a lot of videos and reading a lot on the Web about all of this and it maybe mixing me up a little bit.

When you say Control Rods are they the Rods with Uranium in them?

Fuel rods contain uranium (usually enriched in U-235 with U-238), but the could contain Pu isotopes, e.g., Pu-239 and Pu-241, which are fissile, and Pu-240, which is fertile. Pu produced in reactors inherently contains Pu-239, -240, -241, -242. There are also other transuranics such as isotopes of Am and Cm.

Control rods contain no uranium. Instead they contain B (usually enriched in B-10), Ag-In-Cd (AIC alloy), hafnium or dysprosium. The Russians use Dy titanate, but in the US and EU, B-10 or AIC are typically used.

The fuel rods are clad in Zr-alloy cladding. The control rods are clad in stainless steel, either a AISI 304L or 316L.

I have doen a lot more reading and thought it would be best if I just ask each thing I do not understand then jumpping around.

And this way you can help me little by little undersatand things.

Fael Rods have Uranium Oxide Pellets in them.
And the Fael Rods are 12 Feet Long.
And the Core is made up of Thousands of Fael Rods and 20 Control Rods.
And the Control Rods have Boron in then to Obsorb the Uranium coming from the Fael Rods to help Control the Heat.

I have doen a lot more reading and thought it would be best if I just ask each thing I do not understand then jumpping around.

And this way you can help me little by little undersatand things.

Fael Rods have Uranium Oxide Pellets in them.
And the Fael Rods are 12 Feet Long.
And the Core is made up of Thousands of Fael Rods and 20 Control Rods.
And the Control Rods have Boron in then to Obsorb the Uranium coming from the Fael Rods to help Control the Heat.

Just tell me these fue things befor I go on?

Please try to use correct spelling and proper grammar.

Fuel rods in LWRs, the typical system for commercial nuclear power plants, use fuel in the form of ceramic UO2 or (U,Pu)O2. The ceramic fuel is in the form of right circular cylinders, or pellets, typically with L/D of 1.2.

Fuel rods in most PWRs in the US and older units (900 MWe) in France have fuel stack (column of pellets) length of 12 feet, or 3.66 m, which defines the core height. Modern PWRs have core lengths of 14 feet, or 4.27 m. BWRs have fuel lengths (core heights) of 12.5 ft, or 3.81 m.

The number of fuel rods in a core is determined by the fuel assembly lattice design and the number of fuel rods per assembly. Standard 17x17 fuel assemblies have 264 fuel rods per assembly, but most reactors using 17x17 fuel have 157 or 193 fuel assemblies. More modern units have 205 assemblies and some have 241 or 257 assemblies. BWR have many more assemblies - typically 560 or 764 assemblies in BWR/3 or BWR/4 models, but some have more or less depending on vintage and capacity design.

In PWRs, each assembly is design to accommodate a control rod, but not all assemblies sit under a control. In a core of 193 assemblies, there are 53 control rods. In a BWR, there is approximately 1 control rod for every 4 assembles - there are some assemblies out at the core perifery which do not have a control rod. Those are very low power locations, which do not contribute to the reactivity of the core.

Uranium and boron absorb neutrons. In uranium, neutrons may induce fission, but they may also be captured with a transmutation reaction. Boron absorbs neutrons in competition with uranium, and that is what controls the fission chain reaction. Otherwise, structural materials absorb neutrons in the core, or the neutrons leak out of the core and are absorb by the structural materials surrounding the core.

Exactly.
As Astronuc laid out, a reactor core includes multiple assemblies of multiple fuel rods, each of which contains multiple uranium oxide fuel pellets.
So it is a complex structure whose manufacture mandates exceptional quality control standards to function reliably over multiple decades.

Attached Files:

Ok thanks for the photo I think I get it now.
Every Reactor will be made diferant but for this one each Bundle will be 17x17 Fuel Rods.

Now in the Photo every so often I see a big whole inbetween the Fuel Rods.

It says this is a Watter CH. so would I be right in thinking this?

The whole Bed of 17x17 Fuel Rods are in one Bundle and get submerged under watter. And in the photo when I see the big whole inbetween say every other Fuel Rod is for watter to come all over the Fuel Rods?

Ok thanks for the photo I think I get it now.
Every Reactor will be made diferant but for this one each Bundle will be 17x17 Fuel Rods.

Now in the Photo every so often I see a big whole inbetween the Fuel Rods.

It says this is a Watter CH. so would I be right in thinking this?

The whole Bed of 17x17 Fuel Rods are in one Bundle and get submerged under watter. And in the photo when I see the big whole inbetween say every other Fuel Rod is for watter to come all over the Fuel Rods?

Or am I missunderstanding?

PWR fuel assemblies do not contain water rods or water channels, but rather they have 'guide tubes' which perform several functions:

1. Accommodate the control rods when inserted in the core,
2. Form the structural skeleton of the fuel assembly, and support the spacer grids which maintain the reqular square lattice of the fuel rods,
3. Contain water when control rods are withdrawn, so do provide some moderation in the fuel assembly.

In a given PWR core, all the fuel assemblies have the same lattice. The control rod assemblies are a fixed part of the reactor design since the hardware is fixed in the upper guide structure, which is part of the upper head structure.

In BWR fuel, there are three principal manufacturers (AREVA, GNF, Westinghouse (formerly ABB-Atom)), each having a unique design.

AREVA's ATRIUM design has a large square water channel in the center of the fuel assembly. It is centrally located in the 9x9 and 11x11 designs, but slightly offset on the diagonal oriented toward the control rod in the 10x10 design.

GNF has the GE14 and GNF2 designs which incorporate two cylindrical water rods in the lattice.

Westinghouse's 10x10 is the SVEA-96/-100 design, which is a 10x10 lattice. It has a unique water cross design which sits among four mini-bundles.

All BWR assemblies are encompassed in a square (with round corners) channel which serves to direct the flow up through the assembly (and prevents crossflow). The channel is required because of the significant boiling in the fuel assemblies,

Well I thought every Fuel Rod assembly gets submarged in a Tank of watter?

One big Tank of watter and then the Fuel Rod assembly is submarged?

So why does every Rod need a watter Tube or CH. the watter in the Tank just goes around every Rod anyway?

Or am I missing something?

Water is both a coolant and moderator. Light water reactors use pressurized water for cooling and moderation. In PWRs, the pressure is approximately 2275 psia or ~15.7 MPa (at the core inlet). BWRs operate at 1055 psia (~7.27 MPa).

In BWRs, modern fuel designs incorporate water boxes or water channels within the lattice to improve moderation in the interior of the fuel assembly. This enables a reduction in maximum power peaking in the fuel assembly lattice.

In a fuel assembly, the array of fuel rods forms water channels, i.e., the water flows along the space among the fuel rods. In a BWR, where boiling occurs in the assembly, the water channel allows water to flow up through the assembly without turning to steam.

Attached in an NFI BWR fuel assembly design. The water channel is offset from center in the assembly.